UV texturing mesh object won't turn black in texture painting mode
It's likely that you don't have texture slot to paint on (warning about that and ways to correct are in the top of the T panel):



You have assigned material, set shading to GLSL, set Viewporrt shading to Texture and turned on Texture Paint. Thus the pink color - Blender's warning regarding to leak of texture, wasn't shown. If you switch back to Solid shading, you can see that the mesh is pink (tbh, this was the only way I could avoid of getting pink color in the beginning of the Texture painting). See why the textures are pink, or why the mesh is pink in Texture Paint.

I think it's quite handy to start this work being in the Solid Viewport shading. Then if image is missing you can notice that, while you'll see the texture in all details once it's added: